In general, in an electrophotographic technique or an electrostatic recording technique, a latent image bearing member, which is in the form of a photoconductor or a dielectric, is charged by corona charging or another process. The charged latent image bearing member is exposed by using a laser or a light emitting diode (LED), thereby forming an electrostatic latent image on the latent image bearing member. The formed electrostatic latent image is visualized by, for example, reversal development using a developer such as a toner, so that a high-quality image is formed. In general, a toner used in such a development method is produced through the following processes: mixing a thermoplastic resin as a binder with a colorant, dye as a charge-controlling agent, pigment, and a releasing agent; and processing the mixture into particles having an average particle diameter of 5 μm or more and 15 μm or less through, for instance, kneading, pulverizing, or classifying. Fine particles such as silica or titanium oxide are typically added to toner base particles for the following purposes: imparting flowability to the toner, controlling electrostatic charge of the toner, and enhancing the cleanablity of the toner from the latent image bearing member.
To enhance image quality, the diameter of such toner particles has been reduced. The toner particles having small diameter, such as particles having a diameter smaller than 7 μm, improve the reproducibility of fine lines.
The toner particles with small diameter often contain ultrafine powder having a particle diameter of 3 μm or less. In the case where the toner contains the ultrafine powder having a particle diameter of 3 μm or less, the ultrafine powder may contaminate a developing sleeve. In other words, although the toner is supplied from the developing sleeve to a photoconductor in the developing process, the ultrafine powder may not be supplied onto the photoconductor and stay on the developing sleeve. The ultrafine powder has strong adhesiveness to the developing sleeve. If the ultrafine powder is repeatedly left on the developing sleeve in the developing process, more and more ultrafine powder having strong adhesiveness remains on the developing sleeve. When a thin layer of the toner containing the ultrafine powder is formed on the surface of the developing sleeve, developability may be decreased.
When printing is performed for a long period of time, the ultrafine powder contained in the toner may also adhere onto a surface of a carrier and may cause toner spent. In long-term use of the toner with the ultrafine powder, such problems as fogging in formed images and scattering of the toner from a developing device may be therefore easily caused. Such problems caused by the toner with the ultrafine powders can frequently occur in use of a pulverized toner which is produced as a result of melt-blending a binder resin with other components such as a colorant, releasing agent, and charge-controlling agent and then pulverizing and classifying the resultant mixture.
In addition, the toner particles having a small diameter may pass through a device for removing a residual toner, such as an elastic blade, in a cleaning portion. The residual toner which has passed through the cleaning device may cause a defective image.
In order to overcome the above problems brought by the pulverized toner with particles having a small diameter, for example, a technique has been proposed, in which toner particles with an aspect ratio of 0.8 or more and 0.9 or less are used to form images.
However, even if the toner particles with an aspect ratio of 0.8 or more and 0.9 or less are used, contamination of the developing sleeve and toner spent on a carrier may still be caused. Even if an average aspect ratio of the toner particles is, for example, 0.8 or more and 0.9 or less as a whole, particles of the toner within a certain diameter range may have a smaller aspect ratio. In this case, the particles with a smaller aspect ratio may strongly adhere to the developing sleeve and carrier with its plane parallel to the long axis of the particles. In the case where the particles with a smaller aspect ratio adhere to the latent image bearing member with its plane parallel to the long axis of the particles, the particles may pass through a device for cleaning a residual toner, particularly in a process of cleaning a residual toner.
Moreover, the particles with a smaller aspect ratio have difficulty to be removed from the latent image bearing member (photoconductive drum). A dropout is therefore likely caused in a process of transferring a toner image from the latent image bearing member, with the result that a defective image is formed. The dropout refers to a phenomenon of a defective image in which part of the center of a thin line is not transferred in the transfer process and a formed image has reduced image density at a portion corresponding to the center of the thin line as compared with the image density around the defective area.